Referring first to FIGS. 1A, 1B large frack water storage tanks 1 are assembled using steel panels 2 that may stand twelve feet high. Clamps 3 hold the panels 2 together. A plastic liner 4 (nominally polyethylene or polypropylene over a geotextile underlay garment) holds the water 5, thus preventing leakage through seams 6. Multiple fill/drain hoses 7, 8 are draped over the top edge 9 of the tank 1. To fill the tank 1 a truck with a pump or a pump fed by a pond is connected to a hose inlet 10. Arrows IN on hose 8 shows a fill operation. Arrows OUT on hose 7 show an empty operation which also requires a pump (not shown). Thus, energy is required to empty the tank. Also some water at a height d1 cannot be pumped from the tank 1. An expensive vacuum operation is needed to completely empty the tank 1 when the tank is to be relocated. Valves 11 are used to control the fill/drain operations, these tanks 1 can vary anywhere from 4500 to 41,000 barrels.
What is needed in the art is a fill/drain system that reduces energy and labor costs to the operator. The present invention provides a trench under the panel 2 to use headwater pressure to completely empty the tank 1. The hoses are eliminated. Thus, labor, energy and materials costs are reduced as well as gaining a benefit of reduced time to tear down a large tank.